Populations of thin, motile processes found covering the surface of ciliates (ciliophora) or the free surface of the cells making up ciliated epithelium. Each cilium arises from a basic granule in the superficial layer of cytoplasm. The movement of cilia propels ciliates through the liquid in which they live. The movement of cilia on a ciliated epithelium serves to propel a surface layer of mucus or fluid. (King & Stansfield, A dictionary of genetics, 4th ed)